Six at the top means:

He gets his head in the water. Danger.


Here in conclusion another warning is added. After crossing a stream, a man's head can get into the water only if he is so imprudent as to turn back. As long as he goes forward and does not look back, he escapes this danger. But there is a fascination in standing still and looking back on a peril overcome. However, such vain self-admiration brings misfortune. It leads only to danger, and unless one finally resolves to go forward without pausing, one falls a victim to this danger.